The present invention relates to commercial and/or residential heating and/or cooling systems and other indoor comfort systems, and is more particularly concerned with a furnace control circuit for a forced-air furnace. The invention is more specifically directed to safety features in a furnace control system which respond to possible malfunctions such as a fault condition of a blower or of an inducer.
Forced air furnaces, e.g., gas-fired furnaces, cycle on when the associated thermostat sends a call for heat, and then cycle off when the thermostat is satisfied, so as to maintain the temperature in a desired range within an occupied comfort space.
Whenever the thermostat senses that the temperature is below a set point, the thermostat closes to supply thermostat power to the furnace controller, which in turn commences a heat cycle. Typically, this turns on the inducer blower to induce a flow of combustion air through the furnace heat exchanger. Then a gas valve is opened, and an igniter is actuated. A flame sense device, e.g., a flame rectification probe, senses flame presence, and if flame is proved, a blower delay timer is started. After a predetermined blower delay time, e.g., twenty seconds, the main blower is powered up to force comfort air through the furnace heat exchanger, and supply it through ductwork to the comfort space. When the heated comfort air has sufficiently warmed the comfort space above a thermostat set point, the thermostat opens and cuts off thermostat power to the controller circuit. When this occurs, the gas valve, main blower and inducer follow a shut-off sequence, and the furnace shuts off until a subsequent call for heat.
In some cases, air flow through the forced-air ductwork may become obstructed, such that there is insufficient flow of comfort air through the furnace to pick up the combustion heat in the heat exchanger. A similar condition may occur if the main blower fails to function properly. When this occurs, a temperature sensor in the furnace detects an overheat condition and causes a limit switch to open to cut off thermostat power to the furnace controller and shut the furnace down. In some cases the limit switch must be reset manually, but in some furnace designs, the limit switch may reset automatically after a predetermined delay time sufficient for the furnace to cool. Then, when there is another call for heat, the furnace will undergo another heating cycle and remain on until either the thermostat is satisfied or the limit switch opens again. In systems of this type, unless the obstruction or other problem is cleared, the limit switch will continue to turn the furnace off shortly after it turns on, resulting in a persistent series of unusually short cycles.
In the now-available furnace controllers, there is no functionality provided to track whether a safety limit switch demonstrates a dangerous pattern of excessive cycling or erratic behavior. Moreover, the available furnace controllers do not include any non-volatile or persistent memory device, and any fault indication or other status indication will be lost whenever power is lost to the furnace controller. Forced-air furnaces are required to include a blower-door safety switch that cuts off power if the door to the furnace blower is opened for any reason. All fuel-fired appliances are required to have a door switch to prevent operation of the appliance with the service panel door open. When this happens, the power to the electronics in the furnace controller is also lost, and the microcontroller of the furnace controller loses memory of any information gathered. The current approach to this problem is for the furnace technician to write down the furnace status indications that appear on the furnace controller before opening the blower door, or before otherwise cutting off furnace power. A sight glass is typically provide in the service panel, to allow the service technician to view an LED indicator through the sight glass before opening the panel. However if the sight glass is dirty, the technician cannot see the diagnostic LED indicator. When this is the case, the technician opens the service panel door to get a clear view, and in doing so the door switch opens, power is interrupted and the fault information is lost. It is possible to incorporate a permanent memory feature in the furnace controller circuit so that it retains fault status in the event of a power interruption.
Moreover, if power is interrupted, the lockout state will be cleared when the microprocessor loses power. Then when power is restored, the furnace will be immediately ready to run again, regardless of whether the problem causing the lockout has itself been cleared. This can create an unsafe condition when the controller locks out the furnace but the homeowner resets the power, or when a power outage occurs and the furnace operates again in an overheated condition when power is restored.
It is common for furnace controls to monitor the state of the furnace limit switch, i.e., the temperature sensitive switch on the furnace heat exchanger that opens in the event the heat exchanger temperature exceeds a limit temperature. The limit temperature indicates a fault in the heat exchanger or the ductwork, or may result from inadequate flow of the return air through the fan and filter. When the limit switch opens, the furnace goes through a shut-down cycle and remains off at least for the time the limit switch is open. The limit switch remains open for some fixed, predetermined period, e.g., three minutes. Some furnace controllers monitor the limit switch condition, and if the limit switch opens again after it has reset, software in the controller may lock out furnace operation for some longer, but fixed period of time, e.g., three hours. After the lockout time has expired, as long as there is a call for heat, the furnace will undergo an ignition sequence and again commence supplying heat to the comfort space. Because the conventional furnace controllers only monitor whether faults have occurred, and not the timing of them, they do not adjust the lock-down period to the seriousness of the problem that caused the limit switch to open. A condition that causes the limit switch to open after only a few minutes of furnace operation would be much more serious than a condition that does not result in overheat until after an hour or more. Likewise, temperature faults that occur weeks apart are less significant than temperature faults occurring at much closer intervals. However, because existing furnace controllers do not store the time of occurrence of a temperature fault, they lack the flexibility to match any lock-out period to the seriousness of the problem.
Some modern furnace controllers use an LED to indicate fault problems when they exist, displaying a flash code to indicate the nature of the fault that has occurred. For example, the furnace controller may store the five most recent faults, e.g., temperature fault, pressure fault, flame presence fault, etc. The service technician uses a pushbutton on the controller to browse through the fault codes, one at a time. That is, the furnace controller stores a number of fault messages in memory, and the same controller provides a mechanism for retrieving and displaying the messages, such as using a pushbutton and an LED to blink a flash code that represents the fault message. When the service technician wants to review the messages, he or she presses or taps the pushbutton once, and the LED flashes the first fault message. Then the technician presses the pushbutton again and the LED flashes the second fault message. The technician continues to tap the pushbutton to scroll or browse through the remaining fault messages, one at a time. A problem that comes with this message browsing is that the technician can easily lose track of how many times he or she has pressed the pushbutton, and does not know which message is being displayed. Then if the technician wants to know the content of the fourth message, for example, he or she will have to return to the beginning of the messages and very carefully tap the pushbutton four times. Even then, the technician cannot be certain that the fault message that flashes is the same one that the technician wanted to see. Thus, a need exists for a furnace controller that allows the service technician to view each specific fault message by tapping a simple pattern of taps onto the pushbutton.
Another limitation on now-available furnace controllers is that although they may store a history of operating faults of the HVAC system, they do not record both the type of fault and when it was that each fault occurred. A fault that occurred weeks before would not be particularly relevant to the operation of the furnace, whereas a more recent fault would be much more likely to be the result of a problem or defect in the HVAC system. Unfortunately, the person reviewing the fault history stored on the furnace controller would have no way of knowing when any given fault might have occurred, or the present significance of the fault.
Currently, the furnace controller may store five or more faults in memory, and the controller retrieves and displays the fault history, typically with a pushbutton and LED to blink a flash code. When the service technician wants to review the fault history, he or she presses or taps on the pushbutton. Pressing the button once results in the LED blinking a flash code for the most recent fault; pressing the button again produces the flash code for the second most recent fault, and so forth for the remaining faults stored in memory. Then if the pushbutton is held down for some longer time, such as ten seconds, the fault history is cleared from memory.
As stated above, the service technician will have no way of knowing when any of these faults actually occurred. Often, the technician will lack the patience to browse carefully through all the fault codes, and may not be able to find all the faults. The technician will not know which of these faults in the fault history are actually useful.
For these reasons, a simple list of recorded furnace operating faults, even when in chronological order, does not provide the technician with sufficient information to carry out repair or maintenance tasks effectively.